The biggest objection I have is that one may not withdraw solely from one TSP account or another--distributions must be drawn from both. One chief benefit of having both taxable and tax-free investment accounts is that one may choose the account from which retirement funds are drawn based on the expected tax impact.

By mandating that retirees draw from both, all distributions will contribute to a taxpayer's AGI. No fun if you're trying to minimize taxable income in a given year.